Coeval migmatites and granulites, Muskoka domain, southwestern Grenville Province, Ontario
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We present new field observations and petrologic and geochronological data from the Muskoka domain in the southwestern Grenville Province of Ontario in an attempt to constrain the relationship between amphibolite-facies and granulite-facies gneisses in areas of transitional metamorphic grade, and to examine their implication for tectonometamorphic models for the Grenville Province of Ontario. The predominant medium-grained amphibolite-facies migmatitic orthogneisses of the Muskoka domain contain several generations of leucosome, some of which are related to southeast-directed extensional structures. The amphibolite-facies granitoid gneisses contain numerous mafic enclaves with granulite-facies assemblages recrystallized from anhydrous precursors during Grenvillian metamorphism. Other associated granulites are characterized by their patchy occurrence and gradational contacts, similar to the charnockites in southern India. Patchy granulites, leucocratic vein networks in mafic enclaves, and crosscutting leucocratic granulite veins are interpreted to have formed as a result of local differences in reaction sequences and (or) fluid compositions. The UPb zircon lower intercept age of the patchy granulites overlaps with the previously determined range of 10801060 Ma for high-grade metamorphism in the Muskoka domain, while zircon and titanite from a crosscutting granulite vein crystallized at about 10651045 Ma, supporting a Grenvillian age for granulite formation. Peak metamorphic conditions of 750850°C and 1011.5 kbar (1 kbar = 100 MPa) were determined from the mafic enclaves, whereas the more felsic migmatites reequilibrated at somewhat lower temperatures. The high temperatures caused extensive migmatization and facilitated rheological weakening of the Muskoka domain 1025 million years after the start of the Ottawan orogeny in the Central Gneiss Belt.Keywords:
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Journal Article Zoned Ultrabasic and Basic Gneiss Masses in the Early Lewisian Metamorphic Complex at Scourie, Sutherland Get access M. J. O'HARA M. J. O'HARA Grant Institute of Geology, University of EdinburghScotland Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 2, Issue 2, 1961, Pages 248–276, https://doi.org/10.1093/petrology/2.2.248 Published: 01 June 1961
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The southernmost Hidaka metamorphic belt consists mainly of cordierite tonalite intrusions and pelitic metamorphic rocks ranging from the greenschist to the granulite facies. Anatectic migmatites are common in the higher amphibolite and granulite facies zones. Compositional changes in major, rare earth elements and some other trace metals are so small that they are undetectable among the pelitic metamorphic rocks of zones A + B + C and D, but they are large enough to be detected in the higher amphibolite (zone D) to the granulite facies rocks (zone E). The enrichment of Fe, Mg, Na, Eu, and Sc, and the depletion of K, P, La, Ce, Nd, Cs and Rb are statistically significant in pelitic granulites, while heavy REEs are very variable. The chemical variation of pelitic granulite was derived from the accumulation of plagioclase + garnet. This suggests that more than 50-60% of the total volume of pelitic granulite was melted to produce a large amount of tonalitic magma, leaving pelitic granulite as a restite. Migmatites of the higher amphibolite facies are anatexites, and their K, P, Cs, Rb and light REE content is the same as that of lower grade metamorphic rocks. Migmatites of the higher amphibolite facies melted incipiently to segregate only a small amount of melt, and could not produce a large magmatic mass such as the cordierite tonalites. Cordierite tonalites are S-type granites, and their major elements, Cs, Rb and light REE concentrations are similar to those of lower grade metamorphic rocks. The chemical variation of cordierite tonalites is explained by the extraction of plagioclase + garnet from a tonalitic magma and the variation of original sedimentary rocks. The small chemical difference between the cordierite tonalites and the lower grade metamorphic rocks suggests that the former was derived from a massive melting of metapelites or that much of the restite is retained. The material migration among higher amphibolite facies rocks, pelitic granulites, migmatites and cordierite tonalites took place through mineral/melt interaction in the lower crust.
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Suprasolidus continental crust is prone to loss and redistribution of anatectic melt to shallow crustal levels. These processes ultimately lead to differentiation of the continental crust. The majority of granulite facies rocks worldwide has experienced melt loss and the reintegration of melt is becoming an increasingly popular approach to reconstruct the prograde history of melt-depleted rocks by means of phase equilibria modelling. It involves the stepwise down-temperature reintegration of a certain amount of melt into the residual bulk composition along an inferred P–T path, and various ways of calculating and reintegrating melt compositions have been developed and applied. Here different melt-reintegration approaches are tested using El Hoyazo granulitic enclaves (SE Spain), and Mt. Stafford residual migmatites (central Australia). Various sets of P–T pseudosections were constructed progressing step by step, to lower temperatures along the inferred P–T paths. Melt-reintegration was done following one-step and multi-step procedures proposed in the literature. For El Hoyazo granulites, modelling was also performed reintegrating the measured melt inclusions and matrix glass compositions and considering the melt amounts inferred by mass-balance calculations. The overall topology of phase diagrams is pretty similar, suggesting that, in spite of the different methods adopted, reintegrating a certain amount of melt can be sufficient to reconstruct a plausible prograde history (i.e., melting conditions and reactions, and melt productivity) of residual migmatites and granulites. However, significant underestimations of melt productivity may occur and have to be taken into account when a melt-reintegration approach is applied to highly residual (SiO2 < 55 wt.%) rocks, or to rocks for which H2O retention from subsolidus conditions is high (such as in the case of rapid crustal melting triggered by mafic magma underplating).
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Abstract High‐pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high‐pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high‐pressure granulite in the North Dabie metamorphic core complex in east‐central China, which rarely preserves the high‐pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K‐feldspar + quartz. To investigate the effects of bulk‐rock composition on the stability of orthopyroxene‐bearing, high‐pressure granulite facies assemblages in the NCKFMASHTO (Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 ) system, a series of P – T – X pseudosections based on the melt‐reintegrated composition of the Huangtuling felsic high‐pressure granulite were constructed. Calculations demonstrate that the orthopyroxene‐bearing, high‐pressure granulite facies assemblages are restricted to low X Al [Al 2 O 3 /(Na 2 O + CaO + K 2 O + FeO + MgO + Al 2 O 3 ) < 0.35, mole proportion] or high X Mg [MgO/(MgO + FeO) > 0.85] felsic–metapelitic rock types. This study also reveals that the X Al values in the residual felsic–metapelitic, high‐pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene‐bearing, high‐pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.
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ABSTRACT Suprasolidus continental crust is prone to loss and redistribution of anatectic melt to shallow crustal levels. These processes ultimately lead to differentiation of the continental crust. The majority of granulite facies rocks worldwide has experienced melt loss and the reintegration of melt is becoming an increasingly popular approach to reconstruct the prograde history of melt‐depleted rocks by means of phase equilibria modelling. It involves the stepwise down‐temperature reintegration of a certain amount of melt into the residual bulk composition along an inferred P–T path, and various ways of calculating and reintegrating melt compositions have been developed and applied. Here different melt‐reintegration approaches are tested using El Hoyazo granulitic enclaves (SE Spain), and Mt. Stafford residual migmatites (central Australia). Various sets of P–T pseudosections were constructed progressing step by step, to lower temperatures along the inferred P–T paths. Melt‐reintegration was done following one‐step and multi‐step procedures proposed in the literature. For El Hoyazo granulites, modelling was also performed reintegrating the measured melt inclusions and matrix glass compositions and considering the melt amounts inferred by mass–balance calculations. The overall topology of phase diagrams is pretty similar, suggesting that, in spite of the different methods adopted, reintegrating a certain amount of melt can be sufficient to reconstruct a plausible prograde history (i.e. melting conditions and reactions, and melt productivity) of residual migmatites and granulites. However, significant underestimations of melt productivity may occur and have to be taken into account when a melt‐reintegration approach is applied to highly residual (SiO 2 <55 wt%) rocks, or to rocks for which H 2 O retention from subsolidus conditions is high (such as in the case of rapid crustal melting triggered by mafic magma underplating).
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